I. Field of the Invention:
This invention relates generally to apparatus for measuring the altitude of an aircraft above the earth's surface, and more particularly to a radar altimeter having the capability of essentially simultaneously tracking both the earth's surface and obstructions, such as treetops or man-made objects, extending upward from said earth's surface.
II. Discussion of the Prior Art:
Radar systems are commonly used to implement altimeters for aircraft. Such an altimeter typically includes a transmitter for applying pulses of electromagnetic energy at regular intervals to an antenna which then radiates the energy towards the earth's surface. The altimeter further includes a signal receiver located in the aircraft for picking up echo or return signals reflected from the earth's surface. A closed loop servo tracker for measuring the time interval between the transmitted pulse and its associated return pulse also forms a part of the receiver. The interval is, of course, directly related to the altitude of the aircraft above sea level.
In a military environment, pilots are frequently required to fly missions where they must hug the ground to avoid detection by enemy radar. Prior art radar aircraft altimeters are generally incapable of distinguishing between the earth's surface, be it ground or water, and objects projecting upward from the earth's surface. Specifically, such prior art radar altimeters are incapable of distinguishing between treetops and ground or between man-made objects, such as bridges, towers, etc., and the earth's surface.
It is accordingly a principal object of the present invention to provide a radar altimeter capable of simultaneously tracking multiple targets on a time shared basis allowing both the earth's surface and obstructions projecting therefrom to be indicated on a radar display. This permits a pilot to more safely maneuver at low altitudes over treetops, bridges and mountainous terrain.